Pyrites roasting



Sept. 22, 1931; J. E; LITTLE PY-RITES ROASTING Filed May 4. 192a I I INVENTQR Jalleze 15'. lz'ill'.

TTORNEY Patented Sept. 22, 1931 UNITED STATES PATENT OFFICE JAMES E. LITTLE, OF BETHLEHEM, PENNSYLVANIA, ASSIGNOB TO BETHLEHEM STEEL COMPANY PYRITES nonsrme Application med Kay 4, 1928. Serial no. 274,983.

temperatures of metallic sulphides decrease with a decrease of their particle'size. This may be shown by the following example quoted from page 810 of Lidells treatise 0 Chemical Engineering:

Size of grain in millimeters 1 1-2 Over 2 C'. C. 0. Pyrlte 350 405 47 2 Phyrrhotite 430 525 590 Molybdenite 240 508 Chalcocit 430 679 The difiiculty in roasting finely divided ores on grates, such as are used for burning coarse pyrite, has brought into use furnaces of the multi-hearth type, equipped with rabble arms to continuously stir the material over the hearths, and assist their passage downwards from hearth to'hearth countercurrently to the upward flow of oxidizing gases. In a multiple hearth-furnace, the oxidation of a substantial proportion of the pyrite is elfected as the. fines drop through the discharge opening of one hearth to the hearth next below it; but, in order to limit the velocity of the ascending gases to a degree, at which the free downward passage of the sulphide may not be materially opposed, the volume of the ascendin gases must necessarily be curtailed to within well defined limits.

This limitation is a serious handicap to the,

capacity and efliciency of multi-hearth furnaces, when the latter are operating on pyritic fines.

The burning of fines has hitherto been attended b unavoidable losses of sulphur due to incomp ete oxidation, resulting in a low concentration of S0 in the combustion gases. This fact has been so well recognized in the industry, that pyritic fines command a lower price per. unit of sulphur than the coarser grade of pyrite. In addition, both coarse and fine pyrite command substaptially lower pricesthan those paid for brimstone, since the burning of the latter does not incur the losses attendant upon the burning of pyrite. The availability of pyrite in a condition of fine sub-division, as for example that obtained as a flotation product from pyrite ores, would render this product desirable for burning, if the diificulties above mentioned could be overcome.-

An object of my invention is therefore to effect a substantially complete oxidation of the sulphur present 1n pyrite fines.

A further object of my invention is to attain a high concentration of S0 combustion gases.

A still further object of my invention is to attain an effective control of the combustion conditions, in respect to the feed of pyrite and the amount of excess air necessary for the subsequent chamber or catalytic contact reactions.

I have found that the difliculties hitherto experienced in the burning of pyrite fines,

may be overcome, by bringing the fines into contact with the oxidizing gases (as for exampleair) in a state of suspension and substantial dispersion in -the latter. In this manner, the sulphur, after being dissociated from the iron of the pyrite, may be rapidly and completely converted into S0 gas, while the degree of concentration of S0 in the combustion ases may be controlled by suitably regulatmg the feed of material and the supply of air.

This suspensory and intimate contact of the finely divided pyrite with the air is such as to facilitate a thorough oxidation of the sulphur contained in the former.

The sulphur and iron when submitted to conditions favorable to combustion are first dissociated by heat and, in their resulting nascent state, are each highly susceptible to oxidation by the air supplied for this urpose. The theoretical amount of-air required for burning pyrite is that needed to convert the iron of the pyrite to the ferric state (Fe O,)- in addition to that re uired to combine with the sulphur to form 0 The residue of combustion, viz., the ferric oxide above referred to, drops to the bottom gas in the v of the combustion chamber and may be collected and agglomerated in any convenient manner for utilization in a blast furnace. A certain proportion of the more finely divided oxide may be entrained by the combustion gases and ma be removed therefrom by the usual metho s of precipitating solids from gases, for example, by passing the gases through a Cottrell electric precipitator.

In the manufacture of sulphuric acid by the chamber rocess the S gases should be accompanied by a substantial amount of free oxygen for the purpose of facilitating the oxidizin reactions to be subsequently effected. en burning brimstone, an S0 concentration of 8 to 9% is usually maintained, but when pyrite is used as the source of sulphur, this concentration is lowered to less than 7 thereby involving additional plant capacity and higher operating costs or an equivalent out ut. These several disadvantages may be 0 viated, by burning the pyrite in a condition of sub-division and in suspension, after the manner I have indiecated.

The degree of fineness, most suitable for effecting the proper combustion of pyritic fines, suspended in a current of oxidizing gas,

will somewhat depend upon the character of the ore and the conditions of combustion. In comparing the combustion of pyritic fines with that of pulverized fuel, I have found that, generally speaking, the pyritic fines because of their greater specific gravity should be in a state of sub-division of greater fineness, than is usual when burning pulverized fuel. At any rate, the particles should be fine enou h, so that their complete dissociation and gesulphurization may be effected in their suspended state, so as to result in an adequate formation of S0 and Fe O Particles which are insufiiciently sub-divided tend to drop out of the oxidizing gas stream, and may be recovered unchanged with the residue, any such particles, dropping from the gas stream in an incandescent condition, may be readily observed during the operation.

- The process I have described may be effected in any form of apparatus, in which a controlled amount of pulverized pyrites, pyrite fines, or a pyrite flotation product, may be in'ected into a re ulated stream of air suscepti 1e of entrainmg the particles into a suitable combustion chamber, wherein the articles may become ignited and their com-- ustion completed in a state of suspension, such additional airbeing supplied as may be I, required. Generally speaking apparatus of the kind used for the combustion of pulverized fuel is adaptable to the requirements of my process, and I therefore show in the drawings a preferred form of apparatus of this general character, which embodies the features of my invention.

in any desire In the drawings, I show in:

Fi 1 an elevation in section of apparatus suita le for the burning of pyrite fines, wherein the fines are injected into the combustion chamber in a downward direction and burnt in accordance with the process above described; and

In Fig. 2 I show an elevation in section of 'an alternative type of apparatus, wherein the pyrite fines may be injected in a horizontal direction into a combustion chamber.

Referring to Fig. 1, I show a suitable storage bin 11 for the pyrite fines, a blower 12 suitably connected to the lower part of the bin and an injector 13 to discharge material from 'the bin into a combustion chamber 14. Suitable ports for admitting air into the combustion chamber are shown at 15, and a cinder hole 16 is provided in the lower part of the combustion chamber for the removal of residue or cinder therefrom. A flue 17 is disposed in the upper zone of the chamber to convey gases therefrom into a suitable precipitating chamber 18, as for example a Cottrell electric precipitator. The gases after passing through the precipitator 18 are conveyed to cooling apparatus, of usual design shown at 19, and from thence may be conveyed to a sulphuric acid plant, which may be of the lead chamber or of the catalytic contact type, the forms of said apparatus being well known are not shown in the drawm s.

n the apparatus shown in Fig. 1, the combustion chamber 14 ma be initially heated by building awood fire t erein, or by means of oil burners or in any desired manner. When the combustion chamber has been heated to the proper degree of temperature, the pyrite fines from the bin 11 may be injected into the chamber through the pipe connection 13 under the influence of the blower 12, the rate of feed of the (pyrite being suitably controlled manner. Air in excess may be admitted through the air ports 15, so that the pyrite, upon entering the heated combustion chamber 14 may be ignited and the sulphur of the pyrite partic es may be completely burned to S0 The iron, dissociated fromthesulphur, becomes oxidized and drops to thelower zone of the combustion chamber, whence it may be removed throu h the cinder hole 16. The S0 gases toget er with the oxygen supplied by the additional air admitted through the ports 15, passes through a -flue 17 to the precipitator 18, where any fine particles of iron oxide, entrained from the combustionchamber, may be dropped from the gas stream and removed-from the precipitator at convenient intervals. The gas, from which the solid articles have been removed, thence passes t rough the cooler 19 t o a suitable (plant for the manufacture of sulphuric aci Referring to Fig. 2, I show a storage bin 1 ber to the chamber 28.

for pyrite fines 21, a blower 22 suitably connected to the lower part of the bin, and an inJecting device 23 to convey the yrite fines into a slightly inclined rotary-kiln 24, "of a type wellknown' in the manufacture of cement. At the lower end of the rota kiln, I show a fixed chamber 25, having a dlscharge cinder outlet 26 and air'inlet ports 27.} The lower end of the rotary kiln 24 enters a wall of the combustion chamber and is free to rotate therein. At the upper end. of the rotary kiln I show a precipitating chamber 28 and a gas flue 29 for conveying gases to a sulphuric acid plant.

In this case the pyrite fines may be injected at substantial velocity into a stream of air, traversing the rotary kiln from the cham- Oxidized iron dropped from the gas stream may be gradually conveyed back to the chamber 25 y the rotation of the inclined kiln 24, and may be collected in the lower zone of this chamber and at suitable intervals discharged therefrom through the outlet 26.' The S0 gases and the unburned oxy en from the excess air pass to the precipitating chamber 28, where entrained particles of iron oxide may be removed, and thence the gases pass to a sulphuric. acid plant through the flue 29.

The degree of temperature to which the chamber may be heated, is only such as to initiate the combustion of the sulphur; once initiated, no further extraneous heat is required. Obviously any suitable method for igniting the sulphur will serve the same purpose. In cases where the sulphur content of the pyrite fines is sufliciently high, the excess heat developed by the combustion of the sulphur may serve to effect an agglomeration of the oxidized metallic particles, precipi-' tated from the stream of sulphur dioxide gas. In such cases, apparatus of the ty e shown in Fig. 2 is articularly applicable, or eifecting the nodu izing of the metallic oxides, during their passage down the rotating kiln 24.

From the above it will be seen, that I have devised a method, whereby the sulphur in pyritic fines may be effectively oxidized so as to produce a concentration of SO gas, suitable for the manufacture of sulphuric acid. The method of efi'ecting' contact between the oxidizing gas and the metallic particles, by maintaining the latter in a state of suspension, may be practiced in apparatusof any suitable type, which is susceptlble of fulfilling the conditions disclosed in my process. For purposesof illustration I have shown two types of a paratus therefor.

I have further devised a method, whereby the excess heat developed by the combustion of the sulphur may be used for effecting the agglomeration of the oxidized metallic residue.

While I have shown my invention in .but one form, it will be obvious to those skilled in the art that it is not so limited, but. issusceptible of various other changes and modifications without departing from the spirit thereof, and I desire therefore, that only such limitations shall eplaced thereupon as areimposed by the prior art or as are specifically set forth' in the appended claims.

Having thus described my invention what I claimas new and desire to secure by Letters Patent is: v

1. A process for treating pyritic fines for the purpose of producing sulphurous gases and an agglomerated cinder of metallic oxide, which consists in, blowing the fines into a stream of air, initially igniting the fines suspended in the latter, permitting their combustion to proceed uninterruptedly without application thereto of extraneous heat, controlling the flow of-air so as to supply oxygen more than sufiicient to oxidize both the sulphur and the metals comprising the fines, and utilizing the excess heat developed by the burning of the sulphur to agglomerate the metalhc oxides precipitated from the gas stream.

2. A process for treating pyritic fines for the purpose of producing sulphurous gases suitable for the manufacture of sulphuric acid and an agglomerated cinder of metallic oxide suitable for metallurgical purposes, which consists in, blowin the fines'into a stream of-air, initially igniting the fines suspended in the latter, permitting their combustion to proceed uninterruptedly without application thereto of extraneous heat, controlling th'e flow of air so as to supply oxygen in excess of that required to oxidize both the sulphur and the metals comprising the fines, said excess being calculated to supply the free oxygen requlsite for subsequent conversionof the sulphur dioxide to sulphur trioxide and utilizing the excess heat developed by the burning of the sulphur to agglomorate the metallic oxides precipitated from the gas stream.

In testimony whereof I hereunto afiix my signature.

JAMES E. LITTLE. 

